Tweaking midi tempo. Beats with sligthly different lengths..

[Erik Sjölund]

In swedish folk music, very often the first, second and third beat
have different lengths inside a 3/4 measure. For instance

| a4 b4 cis4 |

could in folk dances be played so the first a4 is 20% shorter in time
than the b4.

What I would like is to specify at one place in the lilypond file this
time unregularity.

firstbeat=1
secondbeat=0.8
thirdbeat=1

and that would be considered for all the meassures in the whole tune
when the midi output is generated.

Is this possible?

cheers,
Erik Sjölund